1. Field of the Invention
The present invention relates to the method of forming a blown plastic article having internal reinforcing ribs. More specifically, the method of forming the article includes blowing a plastic parison into an blown pre-form having a pattern of exterior, outwardly projecting ribs; the preform is then finally blown into the shape of the container whereby the outwardly directed rib pattern is converted into a pattern of internal reinforcing ribs.
2. The Prior Art
In order to save material, plastic bottles which do not require great strength characteristics have been formed with thin wall thicknesses, such as in bottles for half-pint drink cartons. Many of these bottles, however, require reinforcement in the form of externally projecting ribs to compensate for the reduction in wall thickness. Because these grooves are positioned on the outer portion of the bottle, they must be arranged to appear as a decorative effect to aesthetically please the consumer. Accordingly, the external ribs are generally arranged around the entire circumference of the bottle and sometimes along the entire length of the bottle. Such an arrangement results in two primary disadvantages. First, labeling or decorative space is limited or interfered with by the protruding external grooves; and second, greater amounts of material are required to form the ribs to accomplish the decorative effect. The present invention overcomes these disadvantages by the method of providing a bottle having internal reinforcing ribbing (a) which does not interfer with other decoration of bottle, (b) which can be discontinuous so that the bottle can be strengthened only where necessary or desired, and (c) which therefore provide for a material savings.
It is known in the prior art to extrude a tubular plastic parison having external reinforcing ribs which are reversed during the blow molding process to provide internal reinforcement to the blown bottle, as disclosed in U.S. Pat. No. 3,114,932 to Donnelly. It is also known in the prior art to injection mold a plastic parison having a localized thickened region which reinforces the heel of a blown bottle, as disclosed in U.S. Pat. No. 3,137,748 to Makowsky. A primary drawback in the method disclosed by U.S. Pat. No. 3,114,932 is the inability to form circumferential grooves in the plastic parison which are destined to become circumferential reinforcing ribs on the container, without providing a variable extrusion die which is costly because of the required machining, tooling and multiplicity of die parts.
Another problem which can be expected to be experienced in the utilization of the method and apparatus disclosed by the two aforementioned patents is the precise location of the reinforcing portion of the parison at the desired location in the blow molding cavity. During the blow molding procedure of a parison, the plastic material can not be expected to expand uniformly due to localized weaknesses that may appear in the parison itself. As a result, portions of the parison may tend to expand more quickly than other portions of the parison, much like a weakened portion on a inflatable rubber tube. Any such "parison wander" would defeat the objectives of the Makowsky patent and would diminish the desireability of a bottle produced by the process of the Donnelly patent. The present invention further overcomes these problems in the prior art by utilizing a blow molding procedure utilizing a pre-blow and a final-blow, which not only results in a stronger bottle because of molecular orientation, but also affords more precise positioning of any reinforcing ribs due to the relatively small blow-up between the pre-blow station and the final-blow station. The many advantages which accrue from molecular bi-axial stretch orientation are generally known in the art and therefore will not be reiterated here, but reference for such advantages may be made to U.S. Pat. No. 3,767,747, which is incorporated by reference.
Accordingly, the present invention overcomes the disadvantages in prior art bottles having external reinforcing ribbing by providing a bottle which is strengthed by molecular orientation and by internal ribbing. Further, the internal reinforcement ribbing proposed by the present invention may be accurately located through the utilization of a pre-blow and final-blow molding procedure, which necessarily reduces the "blow-up" or expansion of the plastic material after the reinforcing ribs have been formed.